Why do trucks have so many transmission gears?
In: 354
Truck engines are designed to haul heavy loads so they need to create huge amounts of torque so they use diesel engines for their efficiency and large amounts of torque. The problem with diesel engines is they have a very how red line, typically, the larger the diesel engine, the lower the red line as well. The red line is the speed at which an engine stops making more power the faster it turns. As a result of this low red line it has a very narrow power band. A typical car engine will red line at 6000 RPMs, engineers will design the engine to be driveable between a certain rang of RPMs e.g. 1700-4000 RPMs so you can stay in one gear a relatively long amount of time. A typical semi-truck engine will probably have the red line of half that at 2500 RPMs which means but is driveable down to 750 RPMs. This narrow band means each gear has a very narrow range of speeds it can operate at resulting in the need for many more gears.
Semi tractors produce a great amount of torque but do so in a very narrow rpm range, from 1200 to 1500 rpm. In order to get a heavy load moving you need very close ratio gears to be able to keep accelerating and not lose momentum between gears.
When bobtailing ( driving without a trailer ) the load on the motor is low enough that you can skip shift ( skip gears, start in second then shift to 4 then to 6th etc ). When you do that you bog the motor somewhat ( start the gear below 1200 rpm where there is much less torque) but the load is light enough to accommodate. If you try that with a load you won’t have enough power to keep accelerating at such a low rpm and you’d need to shift to a lower gear to accelerate again. The heavier the loads, the more gears you need.
You can think of torque as the amount of force the engine is outputting. You want a lot of torque for hauling heavy loads (as trucks are meant to do) because it allows you to accelerate more easily.
Being in a lower gear gives you more torque, but less top end speed.
In a regular car you may be able to accelerate a heavy load up to 20 mph in first gear while having enough torque. But then when you switch to second gear your torque will be too low and the engine will struggle to accelerate further.
You can accommodate this by spacing the gears closer together. This means that the amount of torque you lose when shifting up a gear will be less. But in turn that next higher gear will not have as high a top speed.
If you space all the gears closer together your top speed in top gear will be too low. The solution is to add even more gears.
In summary. A car has fewer gears spaced further apart to reach a desired top speed. A truck has more gears spaced closer together to reach the same top speed.
Important bit I left out. Your engine generates more torque when your RPMs are in a particular range. Usually something like 4-6k RPM. When you go up a gear your RPM will drop by an amount proportional to the gear spacing. If you have closer spacing your RPM drops less when shifting up. So with closer spacing you can keep the RPM closer to that optimal RPM where it generates the most torque.
Diesel engines have a much narrower power band than gasoline engines. The power band is the RPM range between peak torque and peak power.
The power band on a gasoline engine found in a consumer sedan might be from 1,800 RPM to 6,000 RPM. By comparison, the power band on a semi truck may be 1,400 to 2,200 RPM.
Operation outside of the power band is inefficient, energy from combustion is wasted by heating the engine block or exhaust rather than being transferred to the wheels as motive force.
The high number of gears in freight hauling trucks is necessitated by the need to accelerate a very heavy load from a complete stop, and by the need to keep engine RPM low while at expressway speeds so as to not waste fuel.
It’s much like riding a mountain bike. It’s almost physically impossible to start on the highest/largest gear. Instead, you start with a much lower gear first. This allows you to build up the momentum of the main/pedal gear. Eventually, with enough speed and momentum, the smaller gear becomes too small and you’re pedalling doesn’t make you go any faster.
But now that the main gear has enough momentum, you can change it to a higher/larger gear. Now with each pedal, and much the same force, you go further. Because you’re turning a bigger gear.
Eventually the middle gear isn’t quite enough, but you’re chugging alone nicely and the main gear is turning nicely. So you then change it to the biggest gear.
Trucks are very powerful, but the engine only really provides one turning mechanism. There are so many gears, because there is so many gear sizes that can be used depending on things like the weight of your load, if you’re suddenly going up hill, or if you suddenly need to slow down etc.
Trucks must carry a very heavy load but operate on a diesel engine that has a small operating power rpm range gasoline engines have about a 2000-2500 proper rpm range but large diesels have a 300-500 rpm range.. The only way to get that to while account for uphill and downhill highways is to have a large number of gears. It’s like a 3 banger Geo Metro trying to tow a Suburban with the whole family in it. You would never get out of first because second would not deliver enough tourqe to keep the tires going at that speed without over-revving the engine in 1st. If you add 2 more gears between 1st and 2nd, suddenly you have just enough to keep going.
Short answer: big diesel truck engines only operate from 600-1800 rpm, a 3x range (idle to max operating speed), however a car can range from 600/800 to over 5000 rpm, a 7-9x range, so based on that, if a car transmission has 6 gears, 7 or 9 divided by 3 is 2.33 to 3x,which is how many more gears a truck has, most trucks nowadays have 10-18 gears, most automated truck transmissions have 12.
The way a semi-truck’s gears work are similar to a mountain bike, well not *really* but for this demonstration you can see why. Generally you’ve got 3 gears on the pedals, and somewhere up to 6 on the rear wheel. Now, if you keep both at their minimum then hills, starting off, and even tricks are easier to pull off. Now if you keep the pedal ones at (low) and shift the wheel up to whatever the highest gear is, you’ll notice it takes more to start, but you can go faster for less energy once you do get up to it. If you want to go even faster you might switch the gear on the pedals onto (medium) or (high) and again, it’s harder to move at a stop, but you get more top speed. When you reach a hill, instead of shifting it all back down you might try going back down to (low) and keep pedalling, if you keep your pace it will keep going on smaller hills. Now with this experience in mind, imagine you were pulling a big trailer on that bike full of bricks, or something else heavy, would you want a fixed gear over a set them you can change?
(TL:DR they are there to help the truck move the massive weight down the highway and up hills, and that means *lots* of gears to get up that hill.)
If you look at the technical term for a transmission, it’s device that’s *designed to match engine speed to road speed.*
The gears in the transmission **multiply the output speed of the engine** by a different ratios, so that your speed can increase while the engine speed remains relatively the same.
A lightweight car, with a gas engine, has a **large RPM operating range**. Let’s say you have decent power between 2000 RPM and 6000 RPM, so as your road speed increases, you only need to change a *few* ratios to go between 0mph and 150mph. You can do that with as few as 5 gears.
In a truck, you have a tremendous amount weight AND a very narrow RPM range where the engine makes power. Maybe only between 1500 rpm and 3000 rpm.
In a case like that, you need a lot of very narrow gear ratios even to go between 0-70 MPH.
To move a lot of weight you need a lot of torque. A small gear (the one attached to the engine in this case) can move a big gear (1st gear) easily, but slowly. The engine can only turn so fast, so you have to shift and “shrink” the big gear. If you shrink the size of the transmission gears too quickly, it can bog down and stall the engine.
In the automatic world, i believe most trucks use 6 speeds, so still the same as a lot of cars. In the manual world, the 10 speed is the ‘workhorse.’ Really heavy loads can get you up into the 18-speed range.
Latest Answers